Recent data continue to indicate that DDT use may have adverse effects on human health, but essentially no findings on this topic have been replicated, so the question is still open. Whether DDT has adverse effects in important because DDT is still used for malaria control in 25 countries, alternative pest control strategies with less persistent agents are available, and current recommendations by the World Health Organization that support DDT use are based on the assumption that it has basically no adverse health consequences for humans. DDE is the major metabolite of DDT and is the best long-term biomarker of exposure. Two previous studies suggest that higher exposure to DDE is associated with a shortended duration of lactation. Breast feeding prevents infant death. Therefore, DDT use may have the effect of increasing infant death. Additional information about this potential adverse effect is also needed. Mexico used the insecticide DDT for malaria control until 1999; many there in tropical areas have elevated blood levels of the DDT metabolite, DDE. My project in Mexico has two phases; both are based on the same group of subjects. I conducted phase 1 of this study: 1) To examine the relation between maternal serum levels of DDE in relation to evidence of decreased androgen action in 792 newborn males in Tapachula, Mexico. DDE blocks androgen action in animal experiments. All phase 1 subjects have been enrolled and we are waiting for laboratory results on DDE levels. Phase 2 of this study began at the beginning of FY 2004. This study is following the women and children enrolled in Phase 1, to determine if DDT exposure is related to reduced length of lactation among mothers. In addition, the offspring will be followed to examine early-life DDT exposure in relation to infection and growth. Last year's progress For phase 1, the number of subjects enrolled as of the end of September 2003 was 792, thus the field team more than met the enrollment goal of 750. The response rate was 95%. All specimens for these subjects have been sent to a Canadian laboratory for analysis. Analysis of anthropometric data from the field team shows that the measurements were done reliably and that the measurement error is relatively small compared with the true variation among subjects. I visited both the study coordinating center and the field center twice in the past year. For phase 2, to date, 687 subjects have been followed-up; their breastfeeding duration and status has been ascertained, and the child's growth and infection disease history ascertained. Follow-up of the subjects who are still breastfeeding is ongoing.